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Using advanced MRI techniques, researchers found that obese teenagers tended to have signs of decreased "integrity" in the brain's white matter. White matter contains the fibers that connect different areas of the brain.

In this case, lower white-matter integrity was seen in a brain region related to emotional control and "reward" seeking.

The findings, based on 120 teenagers, are considered preliminary. Experts said it's not clear what they might mean.

But the findings add to evidence linking obesity to certain brain structure differences. Recent studies of middle-aged adults, for example, have found evidence of brain tissue "shrinkage" among those with high levels of body fat -- particularly around the belly.

One possibility is that excess amounts of body fat directly harm the brain through inflammation, the researchers suggested.

In the new study, there was a correlation between decreases in white matter integrity and higher levels of certain inflammatory substances in the blood. Teens with those brain changes also tended to have higher levels of the hormones leptin and insulin. Leptin is involved in appetite control, while insulin regulates blood sugar levels.

Dr. Harold Bays is a fellow of the Obesity Medicine Association and medical director of the Louisville Metabolic and Atherosclerosis Research Center, in Kentucky.

Bays said brain-imaging studies like these provide additional objective evidence that obesity is not just a matter of "willpower."

"Some people don't see obesity as a disease and argue that it's all about behavior," said Bays, who was not involved in the study.

But, he said, obesity is actually driven by a range of underlying factors. "Yes, behavior is a key component," Bays said. "But there are other components, too, including a neurological one."

So does extra body fat cause the brain differences? Or do the brain differences feed weight gain?

Bays suspects there may be a two-way street -- where, for example, brain differences contribute to obesity, and obesity ramps up inflammation that affects the brain.

But this study does not get at that question.

"This doesn't tell you anything about the direction of the relationship," said Allan Geliebter, a senior scientist in psychiatry at Mount Sinai's Icahn School of Medicine in New York City.

It's also unclear, he noted, whether factors other than body fat -- such as diet or lack of physical activity -- could be involved.

Geliebter, who was not part of the study, called it "interesting" in part because it focused on teenagers. If brain differences can be seen that early, that's important, he noted.

Future studies could look at whether such brain differences remain after obese teens lose weight, Geliebter said. That would suggest -- though not prove -- that obesity causes the brain structure changes, he explained.

In a study published earlier this year, Geliebter and his colleagues found hints that this could be the case. They focused on severely obese adults who were starting weight-loss treatment, through surgery or lifestyle changes only. Four months on, patients who were losing weight showed increases in the brain's white and gray matter.

Pamela Bertolazzi, a doctoral student at the University of Sao Paulo in Brazil, is scheduled to present the latest findings Dec. 1 at the annual meeting of the Radiological Society of North America, in Chicago.

Research presented at meetings is considered preliminary until published in a peer-reviewed journal.

For the study, her team used a specialized MRI technique to assess the brains of 59 obese and 61 normal-weight kids, aged 12 to 16. The investigators focused on a measure called fractional anisotropy, or FA: If it's reduced, that suggests lesser integrity in the brain's white matter, Bertolazzi explained.

Overall, the study found, obese teens had a lower FA in certain areas of white matter, compared to normal-weight kids. The affected areas control appetite and emotions.

Bertolazzi said her team hopes to do exactly what Geliebter described -- repeating the MRI measurements in the same teens after the obese group goes through a weight-loss program.

Other studies, she noted, have shown that obese kids tend to have lower IQ scores than their thinner peers, though it's not known whether that is due to any effects of obesity on the brain.

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